The invention concerns a superconducting element comprising a metallic substrate, an insulating layer, a superconductor layer and a metallic protective layer, wherein the insulating layer is disposed between the substrate and the superconductor layer.
A superconducting element of this type is disclosed e.g. in the subsequently published European patent application 12 171 087.5.
Superconducting materials can carry large electric currents without ohmic losses. They are used e.g. in magnet coils or also in resistive and inductive fault current limiters.
Superconducting properties are obtained, in particular, only at comparatively low temperatures that typically require cooling with liquid helium. High-temperature superconductors (HTSL), such as YBCO, have higher transition temperatures and current-carrying capacities in comparison with conventional metallic superconductors such as NbTi or Nb3SN. HTSL are, however, difficult to process due to the brittle material properties. For this reason, they are mostly produced in the form of a strip, wherein a thin HTSL layer is deposited on a flexible substrate.
A sudden transition into the normally conducting state (“quench”) represents one fundamental risk in the use of superconductors. The current that has previously been superconductingly carried in the superconductor then tries to continue to flow through the normally conducting material. This can result in the development of considerable voltages on the quenched superconductor and of heat due to ohmic losses. There is also the danger of irreversible damage to the superconductor (“burnout”).
It is generally desirable for a superconductor to withstand a quench without being damaged. In this respect, superconductors are conventionally protected by parallel normally-conducting current paths with low resistance (“shunt”). In case of a quench, the previously superconducting current can then flow through the parallel current path, thereby reducing the voltage and development of heat. In particular, shunt coatings, e.g. of copper are used as parallel normally-conducting current paths for strip-shaped superconductor structures, as well as metallic substrates, cf. e.g. U.S. Pat. No. 7,774,035 B2. External shunt systems that are coupled via bridge elements are also known, cf. e.g. EP 2 117 056 B1.
The above-mentioned European patent application 12 171 087.5 proposes to select a relatively high internal shunt resistance for strip-shaped superconductors having a relatively short length in relation to the width in order to reduce the danger of a burnout. Towards this end, an interposed electrically insulating intermediate layer separates a metallic substrate from a superconducting film. A thin protective layer of precious metal is disposed on the superconducting film.
EP 2 192 629 A1 discloses a current fault limiter with a superconductor configuration in which the superconducting films of superconducting elements are electrically connected to each other but the electrically conducting substrates of the superconducting elements are insulated from each other. An insulating intermediate layer is arranged between each superconducting film and conductive substrate of the superconducting elements.
US 2012/0040100 A1 discloses provision of a metallic substrate with a Y2O3 layer by means of solution deposition planarization (SDP) including dip coating and heat treatment, to subsequently provide an MgO buffer layer and deposition of a YBCO film on the buffer layer. The SDP treatment of the substrate reduces its roughness and improves the quality of the YBCO film.
It is the underlying purpose of the invention to provide a superconducting element that reduces the danger of damage in case of a quench.